This invention relates to split dies for casting small segments for constituting tread pattern forming portions of tire vulcanizing molds.
Recently, in order to improve operating efficiency and effectiveness of maintenance of tire vulcanizing molds, small pieces or small segments having shapes corresponding to tread patterns are often arranged side by side in width directions of treads and circumferential directions of tires to form tread pattern forming portions, (Japanese Patent Laid-open No. 61-108,512).
As shown in FIG. 1, dies for casting small segments consist as general dies of a lower die 1 and an upper die 2 having a cavity 3 therebetween into which a molten metal such as aluminum is poured to form small segments cast in desired shapes.
In manufacturing these dies, a bottom wall of the lower die 1 must be worked precisely to meet a tread pattern because tread pattern forming portions are formed at the bottom wall of the lower die 1. In other words, a portion of the die in a circle A in FIG. 1 corresponds to a groove of the tread, whose groove corner is particularly to be sharply formed. In the same manner, a portion of the die in a circle B corresponds to a side wall of the small segment which must be worked so as to be a sharply formed groove having a flat wall on one side forming a contact surface with a small segment in assembling. If a contact between two segments is inaccurate to provide clearances therebetween, rubber will penetrate into the clearances in vulcanization to form of fins occurring along the contacting surfaces, which would require a post-treatment similar to that in case of spews.
For producing such dies, the electrical discharge is generally used with electrodes identical in shape with tread pattern forming portions of the dies or separate electrodes for working portions corresponding to grooves and tread surfaces. Therefore, many processes are required for working the electrodes to obtain precise dimensions thereof. In addition, the requirement in manufacturing dies such as sharp working of groove corners above described could not be fulfilled.
With the portion B of the dies shown in FIG. 1, moreover, there is a problem of gas venting to be insufficient in casting so that side walls of small segments could not be sharply formed. As a result, contact between the small segments themselves is often inaccurate.